1. Field of the Invention
The field of this invention generally encompasses radiation induced cell damage and, more particularly, a process of sensitizing tumor cells to the effects of ionizing radiation or protecting nontumor cells from radiation exposure.
2. Description of the Related Art
Radiation of tumor or cancerous cells is a well established and standard therapeutic regimen. Although all of the mechanisms by which radiation affects these cells have not yet been elucidated, certain aspects of radiation induced tumor cell toxicity have been described.
Tumor necrosis factor (TNF), originally identified because of its anti-tumor activity (Carswell et al., 1975), is a pleiotropic biological effector that regulates the activation of genes (Old, 1988; Beutler et al., 1988). Cellular responses mediated by TNF-α include anti-viral and immunoregulatory activity, as well as neutrophil adhesion to vascular endothelial cells. Clinical manifestations of aberrant TNF-α production include cachexia, respiratory distress syndrome and septic shock (Beutler et al., 1988). While TNF-α interacts with two specific cell surface receptors (Tartaglia et al., 1992), many TNF dependent events, including cytotoxicity, are mediated by the 55 Kd receptor, TNF R1 (Wong et al., 1992; Tartaglia et al., 1992; Tartaglia et al., 1993; Tartaglia et al., 1993). The killing action of TNF-α is proposed to occur following receptor binding and production of superoxide and hydroxyl radicals which mediate oxidative damage (Yamauchi et al., 1989; Zimmerman et al., 1989; Old, 1988). The mechanisms responsible for generation of these free radicals are unknown.
TNF-α enhances the tumoricidal activity of ionizing radiation in vitro and in vivo (Hallahan et al., 1990; Sersa et al., 1988; Wong et al., 1991). The recent combination of TNF-α and radiotherapy in a clinical trial has produced encouraging results (Hallahan et al., 1993). However, toxicity from systemic delivery of TNF-α resulted in fever, nausea, loss of appetite, fatigue, lassitude, and hypotension. TNF-α has been selected as the prototype for gene therapy with radiation because this cytokine is reported to have a radiosensitizing effect in tumor cells and either no effect or a radioprotective effect on normal cells. (Hallahan et al., 1990; Sersa et al., 1988; Wong et al., 1991; Hallahan et al., 1993; Neta et al., 1991).
TNF has been shown to signal a variety of responses including cytotoxicity, vascular changes, anti-viral activity, immunoregulation, and the transcriptional activation of genes that participate in homeostasis and inflammation. TNF was first recognized for its ability to induce hemorrhagic necrosis in murine tumors. In addition to the effects of TNF on the tumor vasculature and the cellular immune response, TNF is directly cytotoxic to some tumor cells following binding to TNF receptors. The 55 kd TNF receptor initiates a signal cascade which results in cell death. DNA fragmentation and subsequent cell death from TNF is associated with the production of free radicals such as nitric oxide. TNF cytotoxicity is abolished by the radical scavengers (N-acetyl-cysteine, glutathione and DMSO), anoxia, and the enzyme Mn-superoxide dismutase. Recent data has demonstrated that apoptosis is a mechanism of cell killing by TNF, as well as radiation. DNA is cleaved by endonucleases at internucleosomal linker regions resulting in fragments of multiples of 180 base pairs, and DNA fragmentation precedes cell death (Wright, 1992).
TNF interacts with radiation to enhance killing in some human tumor cells. Synergistic (as defined by decrease in D0) or additive killing between TNF- and x-irradiation was observed in 7 human tumor cell lines. Maximal interactive killing effects between TNF- and x-irradiation were observed when TNF- was added 12 to 4 hrs prior to irradiation. However, interactive effects were absent when TNF- was added after irradiation (Hallahan, 1989, Hallahan, 1990). This synergistic effect is observed in a variety of human tumor cells at a dose that is 10% of that required for cytotoxicity (Hallahan, 1989, Hallahan, 1990). Intracellular reactive oxygen intermediate production has been implicated in tumor cell killing by both TNF and radiation. One other possible means of interactive killing by these two agents is through apoptosis which occurs following exposure of some cells to TNF or radiation. These data indicate that these agents may interact synergistically to kill tumor cells.
TNF-α has been added to cell culture media and the cultured cells exposed to x-rays. Where the target tumor cells exist in proximity to non-targeted cells (e.g., non-tumor cells), the delivery of TNF-α to the target cells will also expose non-targeted cells to that cytotoxin. The problem of non-selective toxicity is even more problematic where the target tumor cells are situated in vivo. The systemic delivery of a cytotoxin results in the exposure of virtually all body cells to that toxin.
Combining TNF with radiation might improve the therapeutic ratio because radiosensitization by TNF is possibly limited to tumor cells. Moreover, TNF radioprotects hematopoietic progenitor cells (Ainsworth, 1959). Exogenous TNF added prior to irradiation has also been demonstrated to protect the hematopoietic system in animals (Sersa, 1988, Neta, 1988). TNF may induce radioprotection through the production of manganese superoxide dismutase (MnSOD), which is associated with enhanced bioreduction of radical oxygen species (Wong, 1988, Wong, 1991). These studies further indicate that TNF may improve the therapeutic ratio when administered during radiotherapy.
In view of the above there exists a need in the art for enhancing the toxic effects of radiation on tumor cells while at the same time minimizing any adverse secondary effects of radiation on non-tumor, non-targeted cells.